Liquid-crystal displays (LCDs) are widely used in projection displays for large screen televisions and monitors. In these LCD-based projection systems, a high power beam of light is passed through a polarizer before being incident on a LCD panel. The LCD panel controls the polarization of the incident light pixel-by-pixel and redirects it towards the corresponding polarizer/analyzer, which then redirects light having the proper polarization to a projection lens that projects an image onto a screen.
One particularly successful LCD-based projection system is a WGP-based LCoS microdisplay system, which uses both wire grid polarizers (WGPs) and liquid crystal on silicon (LCOS) panels. This microdisplay system, which has been proven to exhibit both high resolution and high image contrast when compared to other microdisplay technologies such as transmissive liquid crystal (xLCD), digital light processor (DLP), and direct-view LCD, typically uses three or more microdisplay panels (e.g., one for each primary color band) to improve on-screen brightness.
Referring to FIG. 1, a conventional 3-panel WGP-based LCoS microdisplay system is shown. The microdisplay system includes a light source 5, which for example is a high-pressure discharge lamp, and a light rod 7. The light rod 7 homogenizes the cone of light produced by the light source 5 to ensure a spatially uniform light distribution. Optionally, the light rod 7 is a polarization conversion light pipe (PCLP) for producing linearly polarized light. A first lens 8a passes the light from the light pipe 7 to a first folding mirror 9, which directs the light to a first dichroic filter 10. The dichroic filter 10 separates out the blue light from the remaining light, and directs the blue light via second 8b and third 8c lenses, and second 17 and third 16 folding mirrors to a first LCoS display panel 20a. The remaining light, which is transmitted through the dichroic filter 10, is directed via fourth and fifth lenses 8d and 8e and a fourth folding mirror 11 to a second dichroic filter 12. The second dichroic filter 12 separates the remaining light into green and red light, the former of which is directed to a second LCoS display panel 20b and the latter of which passes to a third LCoS display panel 20c. 
Prior to reaching each LCoS display panel 20a, 20b, and 20c, the incident light first passes through a WGP 15, 14, and 13 and a trim retarder compensator 21a, 21b, and 21c, respectively. Each WGP 15, 14, and 13 is a polarizer/analyser formed from a plurality of parallel micro-wires that transmits light having a polarization orthogonal to the direction of the parallel micro-wires and reflects light having a polarization parallel to the direction of the wires (e.g., if the polarizers are designed to pass horizontal or P—polarized light, as illustrated in FIG. 1, the micro-wires will be perpendicular to the plane of FIG. 1). Each LCoS panel 20a, 20b, and 20c alters the polarization of the linearly polarized incident light pixel-by-pixel and reflects the modulated light back to the corresponding WGP 15, 14, and 13. Since each WGP 15, 14, and 13 is orientated at approximately ±45° with respect to the principal direction of light propagation, in addition to serving as a polarizer/analyzer, each WGP 15, 13 and 14 also serves as a beamsplitter for separating the incoming light from the outgoing light by steering or deflecting the light reflected from the each LCoS panel along an output optical path orthogonal to the incoming optical path. More specifically, each WGP 15, 14, and 13 reflects S-polarized light (e.g., polarized light rotated by 90° by pixels in an ON state) to the X-cube 19. The X-cube 19 aggregates (i.e., converges) the image from each of the three color channels and, via the projection lens 18, projects the final image onto a large screen (not shown). Optionally, each color channel further includes a pre-polarizer (not shown) and/or a clean-up analyzer (not shown), which for example, may include one or more WGPs and/or dichroic sheet polarizers.
The trim retarder compensators 21a, 21b, and 21c (herein simply referred to as trim retarders), are compensating elements used to improve the contrast performance level of the microdisplay system, which is otherwise limited by the residual birefringence of the LCoS panels in the dark (e.g., off) state. In particular, each trim retarder 21a, 21 b, and 21c introduces a phase retardance that cancels the retardance resulting from the inherent birefringence of the corresponding LCoS panel. The term ‘retardance’ or ‘retardation’, as used herein, refers to linear retardance magnitude as opposed to circular retardance magnitude, unless stated otherwise. Linear retardance is the difference between two orthogonal indices of refraction times the thickness of the optical element. Linear retardance causes a phase difference between two orthogonal linear polarizations, where one polarization is aligned parallel to the extra-ordinary axis of the linear retarder and the other polarization is aligned parallel to the ordinary axis of the linear retarder. In contrast, circular retardance causes a relative phase difference between right- and left-handed circular polarized light.
Linear retardance may be described as either in-plane or out-of-plane retardance. In-plane retardance, expressed as optical path length difference, refers to the difference between two orthogonal in-plane indices of refraction times the physical thickness of the optical element. Out-of-plane retardance refers to the difference of the index of refraction along the thickness direction (z direction) of the optical element and one in-plane index of refraction (or an average of in-plane indices of refraction), times the physical thickness of the optical element. Normal incidence rays in a cone bundle see only in-plane retardance, whereas off-axis rays including oblique rays (i.e. non-normal but along the principal S- and P-planes) and skew rays (i.e. non-normal and incident away from the principal S- and P-planes) experience both out-of-plane retardance and in-plane retardance. Notably, in-plane retardance is not observed for the trivial case of 90° ray angle in the birefringent medium.
In the absence of trim retarders 21a-c, the P-polarized polarized light that illuminates each microdisplay panel in the dark (off) state is slightly elliptically polarized upon reflection due to the residual birefringence of the LCoS panels 20a-c. When the elliptically polarized light, which contains both a P- and an S-component, is transmitted to the corresponding WGP 15, 14, 13, the S component is reflected to the X-cube thus allowing dark state light leakage onto the large screen and limiting the contrast of the projection system.
The use of trim retarders 21a-c improves the contrast level by providing in-plane retardance that compensates for the retardance resulting from the residual birefringence in the LCoS panels 20a-c. More specifically, the trim retarders 21a-c are oriented such that their slow axes are configured at orthogonal azimuthal alignment to the slow axes of the LCoS panels 20a-c (termed “crossed axes”), while their fast axes are configured at orthogonal azimuthal alignment to the fast axes of the LCoS panels 20a-c. The terms slow axis (SA) and fast axis (FA), as used herein, refer to the two orthogonal birefringent axes when the linear retardance is measured at normal incidence. Notably, the SA and FA locations change with off-axis illumination as well as reversing the SA/FA roles for a negative out-of-plane retardance component at a large angle of incidence.
Since the slow axes of the trim retarders 21a-c and LCoS panels 20a-c are configured at orthogonal azimuthal orientations, the role of the fast/slow axes switches from the trim retarder 21a-c to the LCoS panel 20a-c for normal incidence light. In other words, light having a specific polarization is alternately delayed more then less, or vice-versa, in the trim retarder 21a-c and the LCoS panel 20a-c , respectively. The net effect is zero relative delay for the incoming polarization, and as a result, an unchanged polarization (i.e., the output light is not elliptically polarized). The corresponding WGP 15, 14, 13 and/or optional clean-up polarizer then rejects the output light so that the dark-state light leakage does not appear on the screen. Since the trim retarders 21a-c do not alter significantly the throughput of the panel on-state, the resulting sequential contrast (full on/full off) is excellent.
In addition to providing in-plane retardance, it is common for trim retarders 21a-c to also provide out-of-plane retardance to increase the field of view. More specifically, it is common for trim retarders to include both an A-plate compensation component for compensating the in-plane retardance and a—C-plate compensation component for compensating for out-of plane retardance. Optionally, trim retarders 21a-c also include an O-plate component. An A-plate is an optical retarder formed from a uniaxially birefringent material having its extraordinary axis oriented parallel to the plane of the plate. A C-plate is an optical retarder formed from a uniaxially birefringent material having its extraordinary axis oriented perpendicular to the plane of the plate (i.e. parallel to the direction of normally incident light). A —C-plate exhibits negative birefringence. An O-plate is an optical retarder formed from a uniaxial birefringent element having its extraordinary axis (i.e., its optic axis or c-axis) oriented at an oblique angle with respect to the plane of the plate.
As discussed above, each trim retarder 21a-c ideally provides an A-plate retardance that matches the in-plane retardance of the corresponding LCoS panel 20a-c in the off-state. In practice, however, the A-plate retardance of both the LCoS panels 20a-c and the trim retarders 21a-c tends to vary within each component due to manufacturing tolerances in device thickness and material birefringence control, as well as operational drifts (temperature, mechanical stress etc). As a result, to ensure adequate compensation it is common to provide a higher A-plate retardance in the trim retarders 21a-c than that exhibited by the LCoS panels 20a-c. For example, a trim retarder with an A-plate retardance of 5 nm (at λ=550 nm) is often provided to compensate for a vertical aligned nematic (VAN) LCoS exhibiting a 2 nm A-plate retardance (at λ=550 nm).
As is known to those of skill in the art, this mismatch in A-plate value requires offsetting of the optic axis of the trim retarder 21a-c, relative to the nominal crossed axes configuration described above. In other words, the trim retarder is ‘clocked-in’ by rotating its azimuth orientation away from the crossed-axes configuration.
For example, consider a VAN-LCoS, where the slow axis of the panel is typically oriented to be substantially parallel to the bisector of the S- and P-planes (i.e., slow axis at ±45° and ±135°, when P-polarization is parallel to 0°/180° and S-polarization is parallel to ±90°). Notably, orienting the slow axis of the VAN-LCoS at ±45° is important if the VAN-LCoS panel is to be used as an efficient electrically-controlled birefringence (ECB) device, the crossed polarization conversion of which is given by:
      I          (              output        ⁢                                  ⁢        crossed        ⁢                                  ⁢        polarization            )        =            I              (                  input          ⁢                                          ⁢          linear          ⁢                                          ⁢          polarization                )              ×                  [                              sin            ⁡                          (                                                                    2                    ⁢                    Δ                    ⁢                                                                                  ⁢                    nd                                    λ                                ⁢                π                            )                                ⁢                      sin            ⁡                          (                              2                ⁢                ϕ                            )                                      ]            2      where Δnd is the single-pass retardance of the VAN-LCoS panel, λ is the illumination wavelength, and φ is the orientation of the slow-axis relative to the P-polarization. In this configuration, the VAN-LCoS functions approximately as a quarter-waveplate retarder in single pass when the panel is in an on-state.
When the slow and fast axes of the VAN-LCoS panel bisect the S- and P-polarization planes, as discussed above, the over-clocking angle of a higher value trim retarder is calculated from the following equation:
      Over    ⁢          -        ⁢    clocking    ⁢                  ⁢    azimuthal    ⁢                  ⁢    angle    ≈                    cos                  -          1                    ⁡              (                  [                                                    Γ                a                            ⁡                              (                LC                )                                      /                                          Γ                a                            ⁡                              (                TR                )                                              ]                )              2  where Γa(TR) is the trim retarder A-plate retardance and Γa(LC) is the LCoS A-plate retardance.
Referring to Table 1, the calculated over-clocking angles for trim retarders providing 2 to 10 nm A-plate retardance for compensating an LCoS panel exhibiting 2 nm A-plate retardance are shown. Both positive and negative azimuthal offsets are given. In addition, two more azimuthal locations are found in the opposite quadrant (i.e., the listed over-clocking angles ±180°).
TABLE 1Approximate over-clocking angles of the trim retardercompensator/VAN-LCoS pair from thenominal crossed-axes configuration.Over-clocked angle from nominalΓa(TR)crossed axes203±24.14±30.05±33.26±35.37±36.78±37.89±38.610±39.2
In general, it has been commonly believed by those skilled in the art that all four over-clocking azimuthal angles of a given trim retarder will produce nearly identical overall system contrast performance. With the assumption that each of the four over-clocking azimuthal angles produces a local contrast maximum, which does not vary between over-clocking azimuthal angles, it is possible to rotate arbitrarily the trim retarder/VAN-LCoS pair to any quadrant (i.e., as long as the slow and fast axes of the VAN-LCos still meets the requirement of bisecting the S- and P-polarizations). Accordingly, in practice, it is common to arbitrarily select one of the four over-clocking angles for any given LCoS orientation and/or for any given WGP orientation, which then serves as a starting position for a subsequent experimental fine-tuning.
Recently, it has been predicted that different over-clocking angles produce different system contrast levels for TN-LCoS projection systems using a MacNeille polarization beamsplitter (PBS), including a small difference for TN-LCoS projection systems using a WGP (e.g., see J. Chen, M. G. Robinson and G. D. Sharp, “General Methodology for LCoS Panel Compensation,” SID 04, Digest, pp. 990-993, 2004).
More recently, the effect of LCoS orientation on system contrast has been studied for LCoS projection systems using a MacNeille PBS (e.g., see J. Chen, M. G. Robinson, D. A. Coleman, and G. D. Sharp, “Impact of the Orientation of Panel Pretilt Directional and Quarter-wave Plate on LCoS Projection System Contrast,” SID 06, Digest, pp. 1606-1609, 2006).
In these studies, however, only two of the four over-clocking angles were examined (i.e., the over-clocking angles with additional rotation ±180° were neglected). Moreover, while the results indicate that the LCoS pretilt direction has a large impact on system contrast (i.e., the system contrast is different for different orientations of the LCoS panel for a given PBS coating surface inclination and/or for different PBS coating surface inclinations for a given LCoS orientation), they do not provide a solution to the contrast inequality. Unfortunately, this means that LCoS engines having different panel orientations and/or different PBS orientations will exhibit different system contrasts. This is a serious concern for manufacturers which require all products to have a same contrast ratio.